68 STUDIES IN GENERAL PHYSIOLOGY 
substance must be formed in the decomposition of the pro- 
teids which attracts the Musca larve even from a distance. 
The contact-irritability of Musca larve.—lit is a well- 
known fact that Musca larve are inclined to crowd into 
cracks and crevices in the earth, and it is astonishing 
through what small cracks the adult larve can slip. This 
irritability might impress a Darwinian as though the ani- 
mals wished to protect themselves from the light. That this 
contact-irritability is entirely independent of their helio- 
tropism is shown by the fact that these animals crowd them- 
selves under a completely transparent glass plate, even if by 
so doing they have to move toward the light. 
The animals retain this form of irritability even when put 
into a vessel of water, in which they soon die. I noticed 
this phenomenon in feeding tritons with fly larvee. Small 
stones lay on the bottom of the vessel, and the larvee crowded 
themselves under them as eagerly and as skilfully as if 
they had always lived under them. The perniciousness of 
this irritability in the case in question is apparent when we 
remember that it keeps the animals from reaching the sur- 
face of the water again, so that they are drowned. 
In these experiments I was struck by the fact that the 
animals, when placed under the surface of the water, do not 
swim upward and so avoid death, but swim downward. I 
cannot explain this fact. Under other conditions positive 
geotropism cannot be demonstrated in these animals. 
The positive heliotropism of flies at the time of seaual 
maturity.—The fly, which as a larva is negatively helio- 
tropic, is positively heliotropic in the state of sexual 
maturity. This reversal in the sense of heliotropism in 
changing to the adult state is not uncommon. Yet it is a 
striking fact that, while heliotropism is reversed, the orienta- 
tion toward chemical substances is the same in the female 
flies at sexual maturity as in the larval state. 
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